Thermoelectric materials are a class of materials that can efficiently convert between thermal energy and electrical energy. The Seebeck effect is the phenomenon underlying the conversion of heat energy into electrical power and is used in thermoelectric power generation. The Peltier effect is related to the Seebeck effect and is a phenomenon in which heat absorption accompanies the passage of current through the junction of two dissimilar materials. The Peltier effect is used in thermoelectric refrigeration or other cooling applications. In addition, thermoelectric materials are used in heating applications and thermoelectric sensing devices.
Only certain materials have been found usable with these effects, which has limited the ability to use this effect.
Some thermoelectric materials are semiconducting or semi-metallic. These materials conduct electricity by using two types of carriers: electrons and holes. When one atom in a crystal is replaced by another atom with more valence electrons, the extra electrons from the substituting atom are not needed for bonding and can move around throughout the crystal. A semiconductor is called n-type if the conducting carriers are electrons. On the other hand, if an atom in the crystal is replaced with an another different atom having fewer valence electrons, one or more bonds are left vacant and thus positively charged "holes" are produced. A semiconductor is called p-type if the conducting carriers are holes.
In the above-mentioned thermoelectric devices, both n-type and p-type thermoelectric materials are usually needed.
Thermoelectric devices can have distinct advantages in many applications. For example, an electric power generator based on thermoelectric materials does not use moving parts like conventional power generators. This feature significantly enhances the reliability of the thermoelectric devices by avoiding mechanical wear of the moving parts and corresponding failure. This further reduces the cost of maintenance. Thermoelectric devices allow operations in hostile environments such as in high temperature conditions (e.g., 900.degree. C.) without human attendance. The unique properties of thermoelectric materials also make the thermoelectric devices environmentally friendly, i.e., industrial heat waste or natural heat sources can be used to generate electric power.
The efficiency of a thermoelectric material is often characterized by a thermoelectric figure of merit, ZT. The figure of merit ZT is a dimensionless parameter and is conventionally defined as: ##EQU1## where S, .sigma., .kappa., and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. The larger the ZT, the higher the energy conversion efficiency of a thermoelectric material. An efficient thermoelectric material should have a large Seebeck coefficient, high electrical conductivity, and low thermal conductivity.
Much interest in thermoelectricity was shown between 1957 and 1963 because of the expectations that a high thermoelectric energy conversion efficiency could be achieved and results transferred to large-scale applications. At that time, bismuth telluride (Bi.sub.2 Te.sub.3) and lead telluride (PbTe) were found among the most efficient thermoelectric materials. Many companies and laboratories were involved in the search for better thermoelectric materials. Later on, Si.sub.1-x Ge.sub.x alloys were added as a prime material for high-temperature space applications. By optimizing the doping level and the composition of state-of-the-art materials, significant improvements were obtained and maximum ZT values close to 1 were reproducibly achieved.
Numerous thermoelectric materials have been synthesized and their properties were investigated. However, the search for materials which combine high electrical conductivity, high Seebeck coefficient and low thermal conductivity did not result in any breakthroughs.
For the entire temperature range of -100.degree. C. to 1000.degree. C., maximum ZT of conventional thermoelectric materials are limited to values of about 1, which were supported by the experimental results achieved at that time. Some workers in the art believed that a ZT of 1 may be a limit common to all thermoelectric materials. However, theoretical attempts to determine such a boundary condition for the dimensionless figure of merit ZT have been unsuccessful so far.
In addition to the low conversion efficiency found in the previous thermoelectric materials, the cost to synthesize these materials is high and thus commercial applications of such devices are often not viable. Furthermore, for the state-of-art thermoelectric materials such as PbTe and Bi.sub.2 Te.sub.3 alloys, the number of isostructural compounds is limited and the possibilities to optimize their properties for maximum performance at different temperatures of operation are also limited.
A systematic search for advanced thermoelectric materials was initiated at the Jet Propulsion Laboratory (JPL) several years ago and resulted in the discovery of a new family of promising semiconducting materials with the skutterudite crystal structure.
Skutterudite structure was originally attributed to a mineral from Skutterud of Norway that has a general formula TPn.sub.3, in which element T can be Co, Rh, or In and Pn can be P, As or Sb. The unit cell of the skutterudite structure (prototype CoAs.sub.3) is cubic space group Im3 and has a square radical [As.sub.4 ].sup.4-. This anion located in the center of the smaller cube is surrounded by eight Co.sup.3+ cations. The unit cell was found to have eight smaller cubes that are often called octants. Two of the octants do not have the anions in the center. This is desirable to maintain the ratio Co: [As.sub.4 ]=4:3 so that the total structure remains electrically neutral and semiconducting. Thus, a typical skutterudite structure results from the Co.sub.8 [As.sub.4 ].sub.6 .dbd.2Co.sub.4 [As.sub.4 ].sub.3 composition and has thirty-two atoms per unit cell.
FIG. 1 shows a typical skutterudite crystal lattice structure. Transition metal atoms 110 form a cubic lattice 112. Non-metal pnicogen atoms 120 form a four-member planary ring 122 which is disposed within the cubic lattice structure 112. Each transition metal atom 110 has six neighboring transition metal atoms 110. Each pnicogen atom 120 has two adjacent pnicogen atoms 120 and two transition metal atoms 110. The covalent bonding associated with a skutterudite-type crystal lattice structure provides high carrier mobility. The complex structure and heavy atoms associated with skutterudite-type crystals also result in relatively low thermal conductivity. These two properties in combination are desirable in improving thermoelectric properties in new semiconductor materials.
Various skutterudite structure materials have been investigated for applications in thermoelectric devices. It is known in the art that high carrier mobility values are usually found in crystal structures with a high degree of covalency. The bonding in a skutterudite structure has been found to be predominantly covalent. Moreover, high hole mobility values have been measured in several skutterudite compounds including IrSb.sub.3, RhSb.sub.3, CoSb.sub.3, and PhP.sub.3.
In addition, thermoelectric materials with a filled skutterudite crystal structure have also been synthesized. The chemical composition of these types of compounds can be represented by the following formula for half of the unit cell: EQU LnT.sub.4 Pn.sub.2 (2)
where Ln includes rare earth elements such as La, Ce, Pr, Nd, Sm, Eu, Gd, Th, and U; T includes transition metal elements such as Fe, Ru, and Os; and Pn includes non-metal atoms such as pnicogen elements P, As, and Sb. The empty octants of the skutterudite, which are formed in the TPn.sub.3 (.about.T.sub.4 Pn.sub.12) framework, are filled with a rare earth element. Because the T.sub.4 Pn.sub.12 groups using Fe, Ru or Os are electron-deficient relative (by 4e.sup.-) to the unfilled skutterudite electronic structure that uses Co, Rh, or Ir, the introduction of the rare earth atoms compensates this deficiency by adding free electrons. However, the number of valence electrons contributed by the rare earth atoms is generally insufficient. For example, La has 3.sup.+ oxidation states, and Ce can be 3.sup.+ or 4.sup.+. Therefore, most of these filled skutterudite compounds behave as metals, or very heavily doped p-type semi-metals.